The present invention relates to a process for stabilizing, by coating, particles of peroxygenated compounds, the particles of peroxygenated compounds thus obtained, as well as washing or bleaching compounds containing such particles.
It is well known that peroxygenated compounds can be used as bleaching compounds in detergent powder mixtures. In the usual household detergents, sodium perborate tetrahydrate is generally used as a bleaching compound, as it is relatively stable to decomposition in a powdered detergent medium. However, it is becoming more and more common to use cold washing and soaking techniques for linen in which sodium perborate has the disadvantage of dissolving too slowly at 20.degree. C.
To rectify this disadvantage, it has been suggested that powdered detergents be used in a mixture with many other peroxygenated compounds, notably alkali metal percarbonates, perphosphates and peroxymonosulfates, which have suitable dissolution rates. These peroxygenated compounds, particularly percarbonates, decompose too quickly in the powdered state, especially if they are stored in a humid atmosphere. Furthermore, the other constituents of the cleaning compounds may activate this decomposition even more.
With a view to avoiding this shortcoming, it has been suggested in French Pat. No. 750125, filed on Nov. 30th, 1932, in the name of Oesterreichische Chemische Werke Gesellschaft, that various types of stabilizing agents be introduced during the manufacture of the peroxygenated compounds by reacting salts or corresponding solid oxides with an aqueous solution of hydrogen peroxide, these stablizing agents being capable of dissolution in the aqueous solution of hydrogen peroxide and/or mixtures in solid phase with the original oxides or salts. The products thus obtained do not, however, have a sufficient stability. Moreover, their active oxygen content is not high.
In order to improve the stability of the peroxygenated compounds, it has also been suggested that the peroxygenated compounds be mixed in the solid phase with metaboric acid, as disclosed in Japanese patent application Ser. No. 49-6760, filed on Sept. 11th, 1970, in the name of Taisho Pharmaceutical Co. The quantities of metaboric acid used are, however, very high and the effect of this is that the active oxygen content of the finished product is low. Furthermore, the stability of the products obtained is not sufficient when they are mixed with other constituents of washing powders.
It has also been suggested that particles of peroxygenated compounds be coated by various types of stabilizing agents such as mixed compounds obtained by crystallization of sodium carbonate with other mineral salts chosen from sodium bicarbonate and sodium sulfate, as disclosed in Belgian Pat. No. 813,645, filed on Apr. 12th, 1974, in the name of Interox. The stability of the granules thus obtained in the presence of the other constituents of the washing powders is very distinctly improved without, however, reaching that of the peroxygenated compounds with a high stability such as sodium perborate when stored for a very long time.